Our Hydro-Hot has the “Domestic Water Priority System”, which disables space heating whenever domestic hot water is being used on a continuous basis.

The result of this feature is that on a cold morning, we get the choice of a hot shower or a warm bathroom. Wash the dishes, and the heat goes off. Worse yet, there is a fairly long delay before the heat comes back on.

I disabled the Domestic Water Priority System by bypassing the Low Temp Cutoff Thermostat a few weeks ago, and we are very happy with the results. Since we are boondocking, we don't take long showers, but so far we can take hot showers while the space heating continues without pause.

Note that the “Domestic Water Priority System” is a feature of all Hydro-Hot models (now renamed to the Aqua-Hot 450) and not the larger Aqua-Hot units found in the Patriot, Marquis, etc. We have the HHE-500-08M Hydro-Hot.

On our 2003 Monterey we can operate the heater and heat the  water at the same time as long as we are using the diesel burner, on the electric side it is one or the other.

Dunno.  Reckon I'd just heat the place up before a shower.  If hooked to 110v., I'd make use of the AC heat pump option, or use a small space heater during the shower.  At least that's the way we've been doing it if ambient temps are really that cold.  It's not the way that smaller HydroHot was designed to work, but I guess the bypass option is okay, as long as the system can keep up with both demands.